Since a polyester resin can result in a coating with excellent film properties such as weather resistance, flame resistance, chemical resistance and the like, it is being watched with keen interest in various technical fields and especially in the automobile industry as a resinous vehicle for a top coat. Usually, a polyester resin is compounded with an aminoaldehyde resin and thus obtained composition is applied and baked to give a tough coating which is excellent in mechanical properties and inter coat adhesion and the like. Nevertheless, such a composition has scarcely been used in an automobile industry as a top coat because the coating has the problems of cissing, loss in gloss and in contrast factor, and heretofore proposed polyester resins each has a problem of poor compatibility with amino resin. Recently, various attempts have been made to improve the properties of an oil-free polyester resin. For example, in Japanese Patent Application Kokai No. 20068/81, a large quantity of saturated alicyclic carboxylic acid or a combination of a saturated alicyclic polycarboxylic acid and an aromatic polycarboxylic acid are used as acid components of said polyester resin, thereby attaining improvements in cissing, loss in gloss and weather resistance of the coating and compatibility of the resin with an amino-plast resin; in Japanese Patent Application No. 155632/82, an alicyclic polycarboxylic acid is used as an acid component and the carboxyl groups capable of developing a resinous acid value are controlled in kind, thereby improving the weather resistance, mechanical properties, intercoat adhesion and curing property of the coating; and in Japanese Patent Applicaition Kokai Nos. 111864/83 and 111865/83, a large quantity of alicyclic polycarboxylic acid is used as an acid component and a comparatively long methylene chain is included in either one of dicarboxylic acid or diol component, or in a lactone to be used for the modification of the formed polyester, thereby improving durability, adhesion properties, elongation and softness of the coating. These studies are noteworthy in a sense that they apply to the practical use of polyester resin, as a resinous vehicle, in an automobile top coat or coating composition to be applied to flexible materials such as fender, bumper and other parts.
In a top coating of automobile bodies and especially in a two-tone-color coating, the main color composition is first applied and baked, and without sanding operation, the second color composition is then applied and baked. At that time, there often arises an interface adhesive failure between the first and the second color coatings. This is especially true when a solid color coating composition is applied on a metallic coat, which combination is most important and in great demand at the present moment. What is worse still, it has been found that the higher the baking temperature of the metallic coat, the poorer the two-tone adhesion.
Under the circumstances, it has long been desired to provide a polyester resin composition for a top coat which is excellent in weather resistance, film properties and curing properties, said composition being suitable for forming a high build coating and to a two-tone color coating and especially recoating of solid color onto a metallic coat, and giving an excellent two-tone adhesion over a wide range of baking conditions.
A principal object of the present invention is therefore, to provide a polyester resin which will fulfill the abovesaid requirements.